The present invention relates generally to the manufacture of coated, bleached paperboard, and more particularly, to the manufacture of such paperboard with a dry finishing technique.
In the conventional manufacture of coated paperboard, the rawstock from the papermachine is surface sized and then machine calendered prior to coating. The surface sizing step is desired to enhance the strength characteristics of the paperboard, to apply a surface film which bonds surface fibers to the body of the paperboard itself and to achieve a holdout for the later applied coating. Meanwhile, the purpose of machine calendering is to achieve a surface smoothness before the coating operation.
In a typical wet finishing process, the effectiveness of the calendering step is enhanced with the application of water from one or more water boxes or the like on the calender stack. The water application imparts good surface smoothness and printability to the paperboard, but not without some detrimental side effects. For instance, when the paperboard is wet finished on the calender, the caliper or bulk of the paperboard is reduced as the density is increased. This decrease in caliper is not ordinarily a problem, except that paperboard is sold according to area and not by weight. Thus, in order to achieve a designated final caliper to paperboard that is wet finished, the rawstock must be manufactured wtih a greater original caliper which increases raw material costs. In addition, wet finishing creates operating problems such as surface mottle of the paperboard due to the non-uniformity of water adsorption of the rawstock, web breaks and corrosion problems on the machine.
Many attempts have been made to manufacture paperboard without wet finishing, however none have been successful in achieving final sheet characteristics comparable to those of a wet finished sheet. The most desirable situation would be a complete elimination of the water application, or simply dry finishing. Dry finished paperboard generally has about 5--10% less density as compared to wet finished paperboard thus requiring less raw material for a given caliper. Furthermore, the elimination of water at the calender reduces the drying load on the machine and other operating problems, thus increasing the operating efficiency of the machine and reducing manufacturing costs. However, to date, it has not been possible to manufacture a dry finished product with the same printing qualities as wet finished board.
In the wet finishing process, the paperboard surface is very responsive to the smoothing action of the calender. One reason for this response is that the water applied plasticizes the surface sizing and the surface fibers, which become pliable, and readily deformed by the pressure at the calender. On the other hand, with the prior art dry finishing techniques, there is no similar plasticizing effect and the surface sizing layer and the surface of the paperboard remain rigid and spongy, so that the paperboard tends to spring back to its original shape once the compression exerted in the calender is removed. In order to improve the prior art dry finishing techniques several counter measures have been tried. One method has included the use of a thermal plastic polymer in the sizing formulation together with hot calendering. Other methods have involved the careful regulation of the moisture content of the paperboard going into the calender and the application of a small, measured amount of water to the board surface during machine calendering in an effort to achieve a good finish without caliper loss. Although each of these methods have resulted in some improvement to the dry finished technique, none has fully realized the economical advantages of dry finishing nor the printing quality achieved with wet finishing.